Remove the inside membrane from the ribs. Whisk the vinegar, ginger, ponzu sauce, sesame oil and garlic together in a large bowl. Add the ribs, turning to coat evenly. Cover and refrigerate for at least 4 hours or up to overnight.
Prepare an outdoor grill for medium-high direct and indirect grilling.
Remove the ribs from the marinade and pat dry. Sprinkle on all sides with the five-spice powder and a few pinches of salt and pepper. Put on the indirect side of the grill, bone side down, and cover the grill. Cook until the meat is tender and the ribs are crisp on the outside, rotating halfway about through, 1 hour to 1 hour and 15 minutes.
Whisk the hoisin and sriracha together in a small bowl. Transfer the ribs to the direct heat side of the grill and continue to cook for an additional 30 minutes, brushing with the sauce about every 10 minutes until the ribs are well glazed. Transfer the ribs to cutting board and let rest 5 minutes. Cut between the ribs and transfer to a serving platter. Scatter the scallions over the top if using.
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